The semiconductor industry continues to develop lithographic technologies which can print ever smaller integrated circuit dimensions. These systems must have high reliability, cost effective throughput, and reasonable process latitude. The integrated circuit fabrication industry has recently changed over from mercury G-line (436 nm) and I-line (365 nm) exposure sources to 248 nm and 193 nm excimer laser sources. This transition was precipitated by the need for higher lithographic resolution with minimum loss in depth-of-focus.
The demands of the integrated circuit industry will soon exceed the resolution capabilities of 193 nm exposure sources, thus creating a need for a reliable exposure source at a wavelength significantly shorter than 193 nm. An excimer line exists at 157 nm, but optical materials with sufficient transmission at this wavelength and sufficiently high optical quality are difficult to obtain. Therefore, all-reflective imaging systems may be required. An all reflective optical system requires a smaller numerical aperture than the transmissive systems. The loss in resolution caused by the smaller NA can only be made up by reducing the wavelength by a large factor. Mirrors are available which provide good reflectivity in the wavelength range between about 13.2 and 14 nm. A reliable long-life light source in this range is desired.
The present state of the art in high energy ultraviolet and x-ray sources utilizes plasmas produced by bombarding various target materials with laser beams, electrons or other particles. Solid targets have been used, but the debris created by ablation of the solid target has detrimental effects on various components of a system intended for production line operation. A proposed solution to the debris problem is to use a frozen liquid or frozen gas target so that the debris will not plate out onto the optical equipment. However, none of these systems have proven to be practical for production line operation.
It has been well known for many years that x-rays and high energy ultraviolet radiation could be produced in a plasma pinch operation. In a plasma pinch an electric current is passed through a plasma in one of several possible configuration such that the magnetic field created by the flowing electric current accelerates the electrons and ions in the plasma into a tiny volume with sufficient energy to cause substantial stripping of outer electrons from the ions and a consequent production of x-rays and high energy ultraviolet radiation. Various prior art techniques for generation of high energy radiation from focusing or pinching plasmas are described in the following papers and patents:
J. M. Dawson, xe2x80x9cX-Ray Generator,xe2x80x9d U.S. Pat. No. 3,961,197, Jun. 1, 1976.
T. G. Roberts, et. al., xe2x80x9cIntense, Energetic Electron Beam Assisted X-Ray Generator,xe2x80x9d U.S. Pat. No. 3,969,628, Jul. 13, 1976.
J. H. Lee, xe2x80x9cHypocycloidal Pinch Device,xe2x80x9d U.S. Pat. No. 4,042,848, Aug. 16, 1977.
L. Cartz, et. al., xe2x80x9cLaser Beam Plasma Pinch X-Ray System,xe2x80x9d U.S. Pat. No. 4,504,964, Mar. 12, 1985.
A. Weiss, et. al., xe2x80x9cPlasma Pinch X-Ray Apparatus,xe2x80x9d U.S. Pat. No. 4,536,884, Aug. 20, 1985.
S. Iwamatsu, xe2x80x9cX-Ray Source,xe2x80x9d U.S. Pat. No. 4,538,291, Aug. 27, 1985.
G. Herziger and W. Neff, xe2x80x9cApparatus for Generating a Source of Plasma with High Radiation Intensity in the X-ray Region, xe2x80x9cU.S. Pat. No. 4,596,030, Jun. 17, 1986.
A. Weiss, et. al, xe2x80x9cX-Ray Lithography System,xe2x80x9d U.S. Pat. No. 4,618,971, Oct. 21, 1986.
A. Weiss, et. al., xe2x80x9cPlasma Pinch X-ray Method,xe2x80x9d U.S. Pat. No. 4,633,492, Dec. 30, 1986.
I. Okada, Y. Saitoh, xe2x80x9cX-Ray Source and X-Ray Lithography Method,xe2x80x9d U.S. Pat. No. 4,635,282, Jan. 6, 1987.
R. P. Gupta, et. al., xe2x80x9cMultiple Vacuum Arc Derived Plasma Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 4,751,723, Jun. 14, 1988.
R. P. Gupta, et. al., xe2x80x9cGas Discharge Derived Annular Plasma Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 4,752,946, Jun. 21, 1988.
J. C. Riordan, J. S. Peariman, xe2x80x9cFilter Apparatus for use with an X-Ray Source,xe2x80x9d U.S. Pat. No. 4,837,794, Jun. 6, 1989.
W. Neff, et al., xe2x80x9cDevice for Generating X-radiation with a Plasma Sourcexe2x80x9d, U.S. Pat. No. 5,023,897, Jun. 11, 1991.
D. A. Hammer, D. H. Kalantar, xe2x80x9cMethod and Apparatus for Microlithography Using X-Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 5,102,776, Apr. 7, 1992.
M. W. McGeoch, xe2x80x9cPlasma X-Ray Source,xe2x80x9d U.S. Pat. No. 5,504,795, Apr. 2, 1996.
G. Schriever, et al., xe2x80x9cLaser-produced Lithium Plasma as a Narrow-band Extended Ultraviolet Radiation Source for Photoelectron Spectroscopyxe2x80x9d, Applied Optics, Vol. 37, No. 7, pp. 1243-1248, March 1998.
R. Lebert, et al., xe2x80x9cA Gas Discharged Based Radiation Source for EUV Lithographyxe2x80x9d, Int. Conf. On Micro and Nano Engineering, September, 1998.
W. T. Silfast, et al., xe2x80x9cHigh-power Plasma Discharge Source at 13.5 nm and 11.4 nm for EUV Lithographyxe2x80x9d, SPIE Proc. On Emerging Lithographic Technologies III, Vol. 3676, pp. 272-275, March 1999.
F. Wu, et al., xe2x80x9cThe Vacuum Spark and Spherical Pinch X-ray/EUV Point Sourcesxe2x80x9d, SPIE Proc. On Emerging Lithographic Technologies III, Vol. 3676, pp. 410-420, March 1999.
I. Fomenkov, W. Partlo, D. Birx, xe2x80x9cCharacterization of a 13.5 nm for EUV Lithography based on a Dense Plasma Focus and Lithium Emissionxe2x80x9d, Sematech International Workshop on EUV Lithography, October, 1999.
Typical prior art plasma focus devices can generate large amounts of radiation suitable for proximity x-ray lithography, but are limited in repetition rate and reliability due to large per pulse electrical energy requirements and short lived internal components. The stored electrical energy requirements for these systems range from 1 kJ to 100 kJ. The repetition rates were typically between a few pulses per second to a few hundred pulses per second and these prior art devices were not suitable for production line operation.
A hollow cathode triggered pinch source which is self-triggered has been described in the literature. However, a disadvantage of this approach is that the output radiation cannot be varied over a significant amplitude range. Adjustment of the output radiation is typically required in laser based lithography sources in order to produce a consistent dose exposure on the wafers and compensate for burst transients associated with the optics in the stepper/scanner as well as energy transients in the source itself. Because the hallow cathode pinch source is self-triggered, there is no rapid means for adjusting the energy delivered to the pinch, and therefore the output radiation energy. The gas pressure inside the device determines the breakdown voltage at which the device triggers and that voltage determines the input energy from the capacitor energy store to the source. In order to vary the output radiation energy, one would have to adjust the gas pressure internal to the pinch. At the high rep-rates required for a production EUV source (for example about 4-5 KH3), adjusting the gas pressure would not be feasible on a pulse-to-pulse basis in order to compensate for these normal energy variations. In addition, timing requirements are likely to exist in synchronizing the hollow cathode triggered source radiation with a radiation energy monitor in the stepper. In a self-triggered system, the variation in timing is likely to be unacceptable.
What is needed is a production line reliable system for producing high energy ultraviolet and x-radiation which operates at high repetition rates with good timing control and avoids prior art problems associated with debris formation.
The present invention provides a pulse power system for extreme ultraviolet and x-ray light sources. The pulse power system produces electrical pulses of at least 12 J at pulse repetition rates of at least 2000 Hz. The system is extremely reliable and has design lifetime substantially in excess of 10 billion pulses. The system includes a charging capacitor bank, a fast charger for charging the charging capacitor bank in time periods of less than 0.5 ms. A voltage control circuit is provided for controlling the charging voltage capacitor to within less than 0.5 percent of desired values. The system includes a magnetic compression circuit for creating, compressing in duration and amplifying voltage pulses. A trigger circuit discharges the charging capacitor bank into the pulse compression circuit so as to produce EUV or x-ray light pulses with a timing accuracy of less than 10 ns. In a preferred embodiment a pulse transformer with at least two one-turn primary windings and a single one turn secondary winding is included in the pulse compression circuit and increases the pulse voltage by at least a factor of 3.
The pulse power system described herein is useful for providing high energy electrical pulses at repetition rates in excess of 2000 Hz for several high temperature discharge EUV or x-ray light sources. These include dense plasma focus devices, Z-pinch devices, hollow cathode Z-pinch devices and capillary discharge devices. Inclusion of the pulse transformer is especially recommended when the system is used for dense plasma focus, and the two types of Z-pinch devices.